The present invention relates to containers for semiconductor devices, more particularly to plastic containers having a metal part and a part of synthetic resin.
This type of container is known under the standardized designation TO-220. It comprises a metal plate which is partially encapsulated within a body of synthetic resin so as to keep a large surface thereof exposed. One or more semiconductor element chips which constitute the active part of the device are affixed to the surface of the plate so as to be in intimate contact therewith, thereby enabling the heat generated by the chips to be dissipated externally during the operation thereof. The part of the plate not encapsulated within the body of resin has a through-hole which enables it to be attached by means of a suitable screw to an external heat dissipator in such a way that the larger surface of the plate without resin is in intimate contact with the heat dissipator.
Rigid metal conductors acting as conducting wires for the device are connected to metallized areas of the semiconductor element chips by means of thin metal wires and they, too, are partially encapsulated within the body of resin.
The above described type of prior art containers, while they have many advantages over other containers of a different construction, such as, for example, the type TO-3 metal containers, do not ensure an equally satisfactory resistance to mechanical stresses in every application.
This drawback, which involves a possible loss of the hermeticity of the container and, thereby, the formation of paths for external agents that are harmful to the active elements of the device, is in part attributable to the fact that, because of the different coefficients of thermal expansion of the metal and the resin, the interface surfaces of contact between the two materials have a tendency to slip on each other when there is a change of temperature, so that the contact between the two materials is not perfect everywhere due to an inadequate mechanical bonding of the body of resin to the metal plate due to deformations that may occur, under thermal stress, of the plate.
It may also be assumed that, immediately after the hardening of the resin body when the bonding of the resin to the plate and the contact between resin and metal are insufficient, the mechanical hermeticity of the container is due only to the bonding of the resin to the semiconductor material of the chips.
Therefore, both during the fabrication steps in the production process following the plastic encapsulation of the container and at the time that the device is being attached to the heat dissipator by the user, there is the danger that the semiconductor chips will be subjected to such mechanical stresses as to cause them to be damaged.
A number of solutions have been proposed in the prior art.
For example, the use of an intermediate layer of a material with good bonding properties both to the resin and to the metal and with a high elasticity coefficient has been proposed, so that the different expansions of the metal plate and of the resin body are compensated for and the stresses to which the semiconductor chips are subjected to are reduced.
The use of brackets, grooves or through-holes on the metal plate has also been proposed with the object of improving the mechanical bonding of the resin body.
However, these solutions do not sufficiently limit the transmission of the mechanical stresses to the semiconductor chips, particularly when the container is attached to dissipators which are not perfectly flat.